1. Field of the Invention
The present invention relates to a method and an apparatus for diametrically contracting a cylindrical body, and more particularly, it relates to a method and an apparatus for diametrically contracting a hollow cylindrical body after the insertion therein of, for example, a honeycomb member of a catalytic converter for an automobile into the hollow cylindrical body. By diametrically contracting the hollow cylindrical body, the inserted honeycomb member is sealingly retained in the hollow cylindrical body.
Note, although the present invention is described hereinafter with reference to a converter for cleaning the exhaust gas of an automobile, for an easy understanding thereof, the present invention is not limited to this usage.
2. Description of the Related Art
With respect to catalytic converter for cleaning the exhaust gas discharged from an automobile engine, the conventional ceramic converter has been replaced by a metallic carrier having a honeycomb structure and made of a heat resisting sheet metal such as stainless steel, as shown in FIG. 9.
In general, this kind of metallic carrier is produced by the following steps:
winding together a flat stainless steel sheet 2a having a thickness of about 50 .mu.m and a corrugated stainless sheet 2b, to form a cylindrical honeycomb member 2; inserting the member 2 into a hollow cylindrical body 3; and soldering or welding the flat sheet 2a, the corrugated sheet 2b and the hollow cylindrical body 3 to each other. In the above-mentioned method, prior to the soldering or welding step, the honeycomb member 2 must be in uniform contact with the hollow cylindrical body 3 to ensure a satisfactory soldering or welding together thereof, as well as a good contact between the flat sheet 2a and the corrugated sheet 2b.
In the prior art method of integrating the honeycomb member 2 and the hollow cylindrical body 3, as shown in FIG. 10, first a metal sheet is formed into a hollow cylindrical body 3 by either pressing or bending. Next, the honeycomb member 2, on which a solder is coated, is placed inside the hollow cylindrical body 3, and thereafter, the body 3 is tightly fastened around the member 2 by metal bands 22. Then, the butting edges of the hollow cylindrical body 3 are welded together with an arc generated from an electrode 23, and finally, a heat treatment for soldering is applied to the hollow cylindrical body 3 having the honeycomb member 2 contained therein.
In the conventional method described above, however, an unequal residual stress may be produced in the honeycomb member 2, due to the fastening of the hollow cylindrical body 3, and thus the member 2 is deformed during the heat treatment. In another case, the welded portion 31 of the hollow cylindrical body 3 is bent in such a manner that a sectional shape of the hollow body 3 does not form a circle. In this case, an outside periphery of the hollow body 3 must be refinished to recover the roundness thereof and meet the required accuracy for an outside diameter of the hollow body 3. Due to this refinishing, however, the thickness of the body 3 is locally reduced, and thus points having a weak mechanical strength are produced on the periphery of the hollow cylindrical body 3.
To solve the above-mentioned problems in the prior art, Japanese Examined Patent Publication No. 57-55886 discloses a drawing method in which a honeycomb member is inserted into a hollow cylindrical body having a diameter larger than the finished product size. Then, as shown in FIG. 11, a drawing tool 24 is drawn in the direction of an arrow F so that the body 3 containing the honeycomb member 2 therein is passed through the tool 24, whereby the outside diameter of the body 3 is contracted and pressed against the honeycomb member 2.
In this method, however, scratches or baking cracks may be produced on the outside of the body 3, due to friction between the tool 24 and the body 3 occurring during the drawing, and thus the appearance of the body 3 is poor due to the presence of such scratches or baking cracks.